Centrifugal pumps generally include a volute provided inside a casing, and an impeller provided on a rotating shaft projecting into the volute. The impeller has a hub mounted on the rotating shaft and a plurality of vanes provided on the hub. The impeller is disposed inside the volute. When the centrifugal pump is to be driven, a self-priming operation is performed to create a negative pressure inside the volute to thereby introduce a fluid, such as water, into the volute by the effect of the negative pressure.
By thus introducing the fluid via the self-priming operation, a steady operation becomes possible. In the steady operation, the fluid is first sucked into the volute, then guided outside the volute (i.e., inside the casing) by the effect of a centrifugal force of the impeller, and finally discharged from the casing to the outside. A typical example of such centrifugal pumps is disclosed in Japanese Patent Application Laid-open Publication (JP-A) No. 03-267596.
When the centrifugal pump disclosed in JP 03-267596 A performs a self-priming operation, a priming fluid is introduced in the volute and the impeller is rotated by driving the rotating shaft. Upon rotation of the impeller, the priming fluid introduced in the volute is guided by the impeller to flow along the volute to thereby force gas (air) in the volute to be discharged outside the volute. As a consequence of this operation, a negative pressure is created inside the volute and, by the effect of the negative pressure, the fluid is sucked into the volute. A steady operation of the centrifugal pump is now ready to be performed.
In a method known as a means for properly discharging gas in the volute to the outside of the volute, the prime fluid in the volute is stirred by utilizing vanes of the impeller. By thus stirring the prime fluid, the gas (in the form of air bubbles) contained in the prime fluid is separated from the prime fluid and discharged from the volute to the outside of the volute.
However, in order to stir the prime fluid in the volute by using the vanes of the impeller, distal ends of the vanes should be formed into a shape which is suitable for stirring the prime fluid. The shape of the distal ends of the vanes greatly contributes to the stirring of the prime fluid. Under these circumstances, the degree of freedom in designing the shape of the vanes' distal ends is considerably low, and sufficient elaborately measures cannot be taken to form the distal ends of the vanes into a shape which is suitable for performing a steady operation.
It is therefore an object of the present invention to provide a centrifugal pump which is capable of stirring a prime fluid and allows vanes to have distal ends formed into a shape suitable for a steady operation.